1. Field of the Invention. This invention relates to a communication system in which a plurality of communication devices communicates with one another through a plurality of transmission lines.
2. Description of the Prior Art. Communication systems, which include many communicating devices located in different places, and which transmit and receive information or signals using a common transmission line in a time division or multiplex manner are known. In general, such communication systems have an auxiliary transmission line in addition to a main transmission line. Typically, the main transmission line is used continuously until it malfunctions. Upon detection of such a malfunction, the system switches from the main transmission line to the auxiliary transmission line.
Thus, during normal periods of operation, the auxiliary transmission line is not used; and, therefore, there is no way of knowing that the auxiliary transmission line is capable of functioning properly. In such systems, malfunctions of the auxiliary transmission line are not detected before a malfunction occurs in the main transmission line, at which time the main transmission line is switched to the auxiliary transmission line. If the auxiliary transmission line is found defective when the main transmission line has malfunctioned, then the system fails since no messages can be transmitted from one device to another over any of the transmission lines available.
In a communication system in which a common transmission line is used in a time division manner, a predetermined communication device will function as the main, or master, station and control the use of the transmission line by the other or slave stations. Such prior art communication systems have the inherent limitation that, if the master station malfunctions, then the communication between the remaining devices ceases even if the remaining devices of the system are capable of operating normally.